BACKPORT: block/loop: set hw_sectors

Loop can handle any size of request. Limiting it to 255 sectors just
burns the CPU for bio split and request merge for underlayer disk and
also cause bad fs block allocation in directio mode.

Change-Id: I15d93462de28a9b25c33f6c38a23a96a6d0f1dc8
Reviewed-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 54bb0ade6627a183c211345761ec46e4bf0048fe)
Signed-off-by: Martijn Coenen <maco@android.com>